Efficiency of nutrient use for biomass production of a <i>Eucalyptus</i> clone as a function of planting density in short-rotation cropping

The objective of this study was to assess the effects of planting density on nutrient use efficiency (NUE) in the production of biomass of a hybrid of <i>Eucalyptus urophylla</i> × <i>Eucalyptus grandis</i>. Twelve planting densities were evaluated at 36 months of age, ranging from 498 to 2564 plants ha⁻¹. Three trees were harvested in each treatment to determine biomass production and allocation (per plant and per area), concentration (g kg⁻¹), amount (kg ha⁻¹), and NUE of N, P, K, Ca, Mg and S in the biomass of leaves, branches, bark and wood. In summary, the increase in planting density resulted in: higher biomass production per area; a reduction in biomass production per plant; a reduction in homogeneity; an increase in the concentration of N, P and K in leaves, of N, Ca, Mg and S in bark, and of N, P and K in wood; a higher amount of nutrients exported by harvesting; and a higher NUE for wood production. The increase in NUE indicates a possible nutritional deficit in trees at higher stocking rates at the age of three years. The higher the planting density, the sooner the site will require re-fertilisation.本研究旨在评估种植密度对尾叶桉（Eucalyptus urophylla）×巨桉（Eucalyptus grandis）杂交品系生物质生产中养分利用效率（nutrient use efficiency, NUE）的影响。本试验共设置12个种植密度梯度，密度范围为498至2564株/公顷，于造林后36个月开展测定。每个处理选取3株桉树，测定其生物质生产与分配情况（单株及单位面积）、养分浓度（g·kg⁻¹）、养分储量（kg·ha⁻¹），以及叶片、枝条、树皮和木质部生物质中氮（N）、磷（P）、钾（K）、钙（Ca）、镁（Mg）和硫（S）的养分利用效率。综合分析表明，种植密度提升会带来以下变化：单位面积生物质产量显著升高，单株生物质产量下降，林分均匀性降低；叶片中氮、磷、钾浓度，树皮中氮、钙、镁、硫浓度，以及木质部中氮、磷、钾浓度均有所提升；采伐作业带走的养分储量增加，且用于木质部生产的养分利用效率升高。养分利用效率的提升表明，在造林后3年的高栽植密度下，林木可能出现营养亏缺。种植密度越高，林地越早需要进行补施肥料。